Excellent mechanical strength and high reliability are required in structures to which a large load is applied, such as mechanical devices, frames (e.g. pipe rack, cable rack, etc.) and construction structures (e.g. architectural structure, bridge, etc.). Accordingly, excellent mechanical strength and high reliability are also required of members constituting these structures such as mechanical devices, frames, construction structures, etc. so that they do not break.
Examples of the member, which is often used for constituting these structures, include fastening members such as a mechanical screw, a bolt with male screw threads and nut with a female screw (referred to as "bolt nut"). The mechanical screw threads or bolt should to be superior in strength in the thin-wall portion (edge) of the screw thread having a shape which is easily broken, and should to have excellent tensile strength and excellent strength against fastening (i.e. high breaking torque) in order to endure an outer force applied to the head portion. In order to satisfy the requirements, metals such as steel, etc. have hitherto been used as a material for the fastening member such as a mechanical screw, a bolt nut, etc.
However, changing the material of the member (e.g. screw, bolt, etc.) from metal to resin has recently been made in view of lightening and/or chemical resistance. As the resin, a vinyl chloride (PVC) resin and a polycarbonate (PC) resin have been proposed.
However, since the PVC resin and PC resin themselves are inferior in mechanical strength, problems such as breakage, tearing, snapping, etc. arise when an excess outer force is applied to a molded article made of these resins. For example, in case of the resin bolt, problems such as breakage due to fastening torque (rupture), breakage of threaded thin-wall portion (edge) and breakage or break-off of a bolt head are liable to arise.
Therefore, for the purpose of improving the mechanical strength of these resin products, a fiber-reinforced resin molded article prepared by molding a composition of a resin and a fiber reinforcement such as glass fibers, etc. has already been suggested.
The fiber-reinforced resin molded article can be roughly classified into a short fiber-reinforced resin molded article and a long fiber-reinforced resin molded article.
The mechanical strength of the fiber-reinforced resin molded article is largely influenced by the affinity of the resin and fiber reinforcement to be mixed. That is, in the combination in which the affinity between the resin and fiber reinforcement is good, the fibers used as the reinforcement are uniformly dispersed in the resin and are also adhered strongly at the whole interface region with the resin and, therefore, an improvement in mechanical strength of the fiber-reinforced resin molded article is realized.
In the short fiber-reinforced resin molded article, the fibers as the reinforcement are easily dispersed in the resin because the fiber reinforcement is easily mixed with the resin due to the shortness of the fibers. Accordingly, the short fiber-reinforced resin molded article has a preferable feature, e.g. improvement of the local mechanical strength of the resulting member occurs because the short fibers fill in (spread over) the thin-wall portion (minute portion).
However, the short fiber-reinforced resin molded article often does not have enough mechanical strength to endure in applications where a large load is applied to the whole molded article, because the fibers are short. That is, the short fiber-reinforced resin molded article had a problem that, when applied to applications to where particularly excellent mechanical strength is required (e.g. fastening member described above, etc.), the mechanical strength (e.g. tensile strength, fastening breaking torque strength, etc.) does not reach a required level.
In case of the long fiber-reinforced resin molded article, the mechanical strength is influenced by the orientation state of the long fibers. Specifically, the mechanical strength of the fiber-reinforced resin molded article is high along with the orientation direction of the blended long fibers and is remarkably improved by uniformly arranging the long fibers.
Accordingly, the mechanical strength of the whole long fiber-reinforced resin molded article can be improved sufficiently because the formulated fibers as the reinforcement are long. However, when the fiber reinforcement is long, the fiber reinforcement can accumulate in the state similar to an integrated material (roving) and, therefore, it is not easily uniformly mixed with the resin. As a result, there often arises a problem that the long fiber-reinforced resin molded article is inferior in mold shape retention, locally (particularly, thin-wall portion, fine structure portion, etc.) and the mechanical strength is not sufficient.
Japanese Patent Kokoku Publication No. Hei 3-25340 (25340/91) discloses a fiber reinforcement-containing resin composition comprising a thermoplastic resin and long fibers, wherein a poor physical affinity of the long fibers and resin is improved.
In this fiber reinforcement-containing resin composition, the physical wetting property (affinity) of the resin for the fiber reinforcement is improved by blending a fiber-reinforced pellet, prepared by arranging reinforcing filaments (fibers) in a low-molecular weight thermoplastic resin in parallel with each other, with a thermoplastic resin having a molecular weight higher than that of the above thermoplastic resin. As a result, the fiber-reinforced resin molded article obtained from this long fiber-reinforced resin composition has sufficient mechanical strength at the portion other than thin-wall portion. However, this fiber-reinforced resin molded article still has a because that the long fiber is not filled in the thin-wall portion such as threaded portion (edge portion) and the mechanical strength of the thin-wall portion is low and, furthermore, the mold shape transferring property at the fine structure portion is poor, because the formulated fibers as the reinforcement are long.
Furthermore, the long fiber-reinforced resin molded article also had a problem that the long fiber is liable to be snapped in the production, particularly in case of melt-kneading the long fiber reinforcement-containing resin pellet and, as a result, the mechanical strength is liable to become lower than an expected value.
In order to solve these problems, a process for the production of a long fiber-reinforced thermoplastic resin composition, wherein the long fibers are prevented from snapping by blending a long fiber reinforcement-containing thermoplastic resin and a thermoplastic resin containing no long fiber, has already been proposed (Japanese Patent Kokai Publication No. Hei 1-241406). According to this process, the fiber-reinforced resin molded article obtained from the composition containing the long fibers as the reinforcement has excellent mechanical strength because the long fibers as the reinforcement in the long fiber reinforcement-containing composition are not easily snapped.
However, even in case of this fiber-reinforced resin molded article, the fibers as the reinforcement do not sufficiently fill the thin-wall and fine structure portion such as a thread, etc. because the fiber reinforcement is composed of the long fibers. Therefore, there still remains a problem that the mechanical strength of thin-wall and fine structure portion is not sufficiently improved and the mold shape transferring property is low.
On the other hand, a fiber-reinforced resin molded article prepared by mixing a combination of short glass fibers and long glass fibers with a resin has already been disclosed (Japanese Patent Kokoku Publication No. Hei 4-65854). This fiber-reinforced resin molded article is a glass fiber-reinforced resin sheet obtained by impregnating a long glass fiber reinforcement mat with a thermoplastic resin (polypropylene) containing a short glass fiber reinforcement, thereby formulating the short glass fibers and long glass fibers in a specific ratio. This fiber-reinforced resin molded article can be formed into a sheet-like stamping molded article (compressed molded article) having excellent strength even in the detail portion by allowing the short glass fibers to spread over the detail portion while maintaining the mechanical strength due to the long glass fiber reinforcement.
However, the shape of this fiber-reinforced resin molded article is limited to a flat shape (e.g. sheet, etc.), which can be stamped, in view of its production process, and it is difficult to make a product having a three-dimensional shape, such as a screw, bolt, etc. In this fiber-reinforced resin molded article, it is most difficult to arrange long glass fibers as the reinforcement constituting the mat in a longitudinal direction of a screw, bolt, etc.
Accordingly, this fiber-reinforced resin molded article could hardly applied to the development of fastening members such as a screw, bolt, etc. which are required to sufficiently satisfy the requirements to the tensile strength and fastening breaking torque strength as well as strength of the thin-wall portion and mold shape transferring property of the fine structure part because it is used under severe conditions.
The first object of the present invention is to solve various problems accompanied with the above prior art as a whole and to provide for a fiber reinforcement-containing polyolefin composition capable of providing for a fiber-reinforced resin molded article which is superior in tensile strength and fastening breaking torque strength as well as strength of the thin-wall portion and mold structure transferring property of the fine structure portion.
The second object of the present invention is to provide for a bar-like molded article formed of the fiber reinforcement-containing polyolefin composition, particularly various bolts. The bolts according to the present invention include a "single-end bolt" provided with a screw thread at one end, a "double-end bolt" provided with a screw thread at both ends and a "continuous-thread bolt" provided with a screw thread over the whole length.